Nowadays, steel sheets which are subjected to a surface treatment and provided with a rust preventive property, in particular, galvannealed steel sheets with excellent rust preventive property, are used as material steel sheets in the fields of, for example, automobile, domestic electric appliance, and building material industries.
Generally, a galvannealed steel sheet is manufactured by performing recrystallization annealing using an annealing furnace in a CGL (continuous galvanization line) on a steel sheet which is obtained by performing hot rolling and cold rolling on a slab, by performing galvanization on the annealed steel sheet, and by performing a heating treatment on the galvanized steel sheet so as to form an Fe—Zn alloy phase due to the occurrence of an alloying reaction in which Fe in the steel sheet and the Zn in the coated layer are diffused. This Fe—Zn alloy phase is a coating usually consisting of a Γ phase, a δ1 phase, and a ζ phase. There is a tendency for hardness and a melting point to decrease with decreasing Fe concentration, that is, in the order of a Γ phase, a δ1 phase, and a ζ phase. Therefore, from the viewpoint of sliding property, it is effective to use a coating having a high Fe concentration with which adhesion is less likely to occur due to its high hardness and high melting point, and thus a galvannealed steel sheet for which press forming performance is important is designed so as to have a rather high Fe concentration in its coating.
However, in the case of a coating having a high Fe concentration, a hard and brittle Γ phase tends to be formed in the interface of a coated steel sheet (interface between the coated layer and the steel sheet), and there is a problem in that a phenomenon in which the coating exfoliates from the interface, that is, so-called powdering, tends to occur when the coated steel is subjected to forming. Patent Literature 1 discloses an example of a method for solving this problem in which a second layer of a hard Fe-based alloy is formed on the coated layer by using an electric plating method in order to achieve both a sliding property and an anti-powdering property. However, since a treatment apparatus such as an electric plating apparatus is necessary to use the technique according to Patent Literature 1, the technique is not preferable from the viewpoint of facility design and economic efficiency, and, in addition to that, an effect of fundamentally increasing an anti-powdering property cannot be expected.
On the other hand, Patent Literature 2 discloses an example of a technique for increasing an anti-powdering property in which a heating rate and a cooling rate are controlled when an alloying treatment is performed. However, in order to control a heating rate and a cooling rate, it is necessary to increase facilities, for example, which results in an increase in cost. In addition, in the case of a method according to Patent Literature 3 where Si content and P content in steel are restricted, it is difficult to achieve the required material qualities of a steel sheet such as high strength and high ductility due to the restriction of Si content and P content.